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Aging Mechanisms in Amorphous GeTe
Raty, Jean-Yves; Zhang, Wei; Luckas, Jennifer et al.
2015MRS spring metting 2015
Editorial reviewed
 

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Keywords :
ab initio; phase change materials; aging
Abstract :
[en] We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or the more elaborate Van der Waals approximation. New insight is provided on the stability of homopolar GeGe bonds and tetrahedral Ge bonding, in relation with the resistance drift phenomenon, that is investigated experimentally using photothermal deflection spectroscopy experiments. Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram. Since the structural relaxations concerned with the drift take place on long time scales, the task of understanding them to limit their consequences is not a simple one. We successfully achieved this goal by developing new approaches to overcome a series of hurdles. A first problem is that directly generating an amorphous structure by quenching a liquid using Density Functional Theory (DFT) based Molecular Dynamics leads to one sample with a small number of atom (typically a few hundreds), and, hence of small number of atomic environments. Here we sample a large number of local atomic environments, corresponding to different bonding schemes, by chemically substituting different alloys, selected to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging. A second aspect is that GST alloys are known to display complex bonding mechanisms, for which the simple chemist’s “octet-rule” does not apply, leading a long series of controversies, concerning in particular the local structure around Ge atoms. We overcome this problem by using state of the art non local DFT-MD, including the so-called van der Waals corrections. This leads to more clearly defined environments that are thoroughly analyzed. We can then identify their fingerprints in the available structural experimental data and assess the stability of these local environments to obtain information of the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments that are presented here. Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal (by getting rid of homopolar bonds), and an evolution of its electronic properties that drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous, as compared to the crystal.
Research center :
Physique des Solides, interfaces et Nanostructures
Disciplines :
Physics
Author, co-author :
Raty, Jean-Yves  ;  Université de Liège > Département de physique > Physique expérimentale des matériaux nanostructurés
Zhang, Wei
Luckas, Jennifer
Mazzarello, Riccardo
Bichara, Christophe
Wuttig, Matthias
Language :
English
Title :
Aging Mechanisms in Amorphous GeTe
Publication date :
08 April 2015
Event name :
MRS spring metting 2015
Event organizer :
Materials Research Society
Event place :
San Francisco, United States
Event date :
6-04-2015 to 10-04-2015
Audience :
International
Peer reviewed :
Editorial reviewed
Funders :
F.R.S.-FNRS - Fonds de la Recherche Scientifique [BE]
FWB - Fédération Wallonie-Bruxelles [BE]
DGTRE - Région wallonne. Direction générale des Technologies, de la Recherche et de l'Énergie [BE]
ULiège - Université de Liège [BE]
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since 29 June 2015

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